As is known in the art, while there is extensive understanding of human skin properties based on active tensile testing, both in vitro and in vivo, there is little current knowledge of the strains experienced by skin during natural movements.
Understanding the skin's material properties and natural motion is important to help provide better understanding in a number of areas including: creating tissue engineering scaffolds that integrate smoothly with little scarring; understanding skin growth; aging; and changes due to diseases. Past work has been completed to ascertain material property data of isolated skin samples with in vitro tensile testing and by applying external loads (pulling, twisting, indenting, and suctioning) and measuring the resulting deformations in vivo. These experimental methods are limited because the skin behaves differently when removed from surrounding tissues in vitro and in vivo techniques do not take into account the skin's biaxial prestress. Because of the anisotropic behavior, it has been theorized that the skin contains lines of non-extension (LONEs), or contours of the skin that stay a constant length with minimal stretching capacity that only rotate during joint motion. As the body moves, particularly close to the joint, these LONEs do not always exist; in those cases the contours that are most important are those with the minimum extension or compression. Previous work, however, have only been able to qualitatively find non-extending lines.